The role of semiconductors in modem computers and electronic devices can hardly be understated. Semiconductor materials make possible basic components such as transistors, diodes, LEDs, lasers, memory, solar cells, and sensors that enable a range of devices and technologies. Most of the tremendous economic impact associated with the information industry is attributable to the development and advances in semiconductor materials and processing with silicon being far and away the most important material. These advances continue today as researchers and companies continue to discover new materials for new applications and new processing technologies. Examples of current activity include blue lasers and emitters as exemplified by GaN and related materials as well as techniques for miniaturizing devices and feature size as exemplified by deep UV and x-ray lithography.
The device capabilities afforded by semiconductors is a consequence of their unique electronic properties. An important feature of a semiconductor is the presence of an electronic and gap between the electronic states of the valence and conduction bands. The electronic band gap represents a range of energy extending from the valence band edge to the conduction band edge in which no electronic states are present in an intrinsic semiconductor. The presence of a band gap provides an ability to exercise substantial control over the flow of electrons within and through a semiconductor. Further control of conductivity can be achieved by including dopants in a semiconductor composition. Dopants are examples of defects that interrupt the regular periodicity of a semiconductor lattice thereby providing a mechanism for introducing electronic states and carriers in the electronic band gap. Other defects include impurities and structural irregularities such as vacancies and interstitial atoms. Electronic states associated with defects provide the n and p type conductivity typically associated with semiconductors. The nature of defects (e.g. chemical identity of dopants or interstitial atoms, specific form of structural irregularity etc.) determines the energy of defect electronic states relative to the valence or conduction band edges. The concentration of defects determines the number of defect electronic states in the band gap as well as the number of electrons that can occupy those states. Since defects can provide occupied and unoccupied electronic states in close energetic proximity to the conduction and valence band edges, respectively, they provide for enormous flexibility in controlling the conductivity and flow of current through semiconductor materials. This flexibility is at the heart of the tremendous latitude afforded by semiconductors for controlling the flow of electrons that is responsible for virtually all of the important effects associated with computers and other electronic devices. In essence, semiconductors provide an ability to precisely control and direct current flow through the control of the electronic band gap, number density of electronic states and charge carriers, energies of electronic states, doping strategies, defects and chemical composition.
Attention recently has been focused on developing materials capable of controlling the propagation of light in much the same way that semiconductors control the propagation of electrons. Over the past decade substantial progress has been made toward this goal and the new field of photonic crystals has emerged. A photonic crystal functions as a “semiconductor for light” in the sense that it may possess a photonic band gap that defines a range of electromagnetic frequencies that are unable to propagate in the crystal. In the most fundamental sense, the ability of semiconductors to control the propagation of electrons originates from the periodic lattice arrangement of the atoms that constitute the semiconductor. The precise arrangement and spacing of atoms ultimately dictates the band structure and electronic states of a semiconductor. Periodicity is also a key concept in photonic crystals. Instead of atoms, however, the periodic building block of a photonic crystal is a macroscopic dielectric medium. An example of a photonic crystal would be a material that consists of a flat dielectric slab that contains a periodic arrangement of small holes aligned along the thin dimension of the slab. Such a material may be viewed as a periodic arrangement of rods comprised of air and corresponds to a photonic crystal in which air is the macroscopic dielectric medium.
The precise details of the periodic pattern of rods and the refractive index contrast between the periodic macroscopic dielectric medium and its surroundings influences the properties of the crystal. In the preceding example, the flat dielectric slab has a high refractive index, while the air holes have a low refractive index. The refractive index contrast of a photonic crystal functions with respect to photon propagation much like the electronic potential of atoms in a semiconductor functions with respect to electron motion. A periodic lattice arrangement of macroscopic dielectric media such as rods provides photonic analogues of many of the electronic properties associated with the lattice periodicity of atoms in a semiconductor. The most important of these analogue properties is the photonic band gap. Light having an energy within the photonic band gap and propagating in a direction defined by the photonic band gap is blocked and unable to propagate in a photonic crystal. When external light having an energy and direction of propagation within the photonic band gap is made incident to a photonic crystal, it is unable to propagate through the crystal. Instead, it is perfectly reflected. Light with an energy or direction of propagation outside of the photonic band gap, on the other hand, freely passes through the crystal (subject, of course to ordinary absorption and reflection processes).
Photonic crystals can be formed from a wide variety of macroscopic dielectric media provided that an appropriate refractive index contrast with a surrounding medium can be achieved. As an example, the composition of the holes or rods in the example above is not limited to air. Other materials that present a sufficiently large refractive index contrast with the surrounding flat dielectric slab may be used to form the rods. A periodic lattice of air holes, for example, may be drilled in a flat dielectric slab and subsequently filled with another material to form a photonic crystal. The rod material may have a higher or lower refractive index than the slab material. As another example, a periodic array of rods comprised of a macroscopic dielectric medium such as silicon in air as the surrounding medium represents a photonic crystal.
Important material design considerations include the size, spacing and arrangement of macroscopic dielectric media within a volume of surrounding material as well as the refractive indices of the dielectric and surrounding materials. The periodicity of the macroscopic dielectric media can extend in one, two or three dimensions. These considerations influence the magnitude of the photonic band gap, the frequency range of light or other electromagnetic energy (e.g. infrared, microwave etc.) that falls within the photonic band gap and whether the photonic band gap is full (in which case the photonic band gap effect is manifested regardless of the direction of propagation of the incident light) or partial (in which case the photonic band gap effect is manifested for some, but not all, directions of propagation). Other practical considerations are also relevant such as manufacturability, cost, ability to fabricate a periodic array of rods etc.
Effects analogous to doping or defects in semiconductors may also be realized in photonic crystals. An inherent consequence of dopants or defects in semiconductors is a disruption or interruption of the periodicity of the lattice of atoms that constitute the semiconductor. The electronic states associated with dopants or defects are a direct consequence of the local disturbance in periodicity imparted to the semiconductor lattice. Photonic crystals can similarly be perturbed in ways analogous to introducing dopants and defects in semiconductors. The periodicity of a photonic crystal is a consequence of a regular and ordered arrangement of macroscopic dielectric media (e.g. rods) within a surrounding medium (e.g. dielectric slab). Effects that interrupt the arrangement of macroscopic dielectric media can be used to break the periodicity to create photonic states within the photonic band gap. Possible ways of perturbing an array of rods in a surrounding dielectric slab, for example, include varying the size, position, optical constants, chemical composition of one or more rods or forming rods from two or more materials. The ability to create photonic states within the photonic band gap provides further flexibility in controlling the frequencies and directions of incident light that are reflected, redirected, localized or otherwise influenced by a photonic crystal.
It is widely expected that photonic crystals will be the basis of next generation information, optical and communication systems. Many people believe that the potential ability to control the propagation of light offered by photonic crystals may exceed the ability of semiconductors to control the propagation of electrons and that a commensurately greater economic benefit will result as new technologies and industries based on photonic crystals that are able to selectively inhibit, direct or localize the propagation of light in increasingly complex ways. The technological areas in which photonic crystals are projected to make an impact continue to grow in scope and kind. Projected applications include LEDs and lasers that emit light in very narrow wavelength ranges or that are of nanoscopic dimensions, direction selective reflectors, narrow wavelength optical filters, microcavitics for channeling light, color pigments, high capacity optical fibers, integrated photonic and electronic circuits that combine photonic crystals and semiconductors to produce new functionality, devices for light confinement, optical switches, modulators, and miniature waveguides.
As the field of photonic crystals develops, the need for new photonic band gap materials is increasing. A much needed feature to enhance the flexibility and range of applications of photonic crystals is an ability to adjust the properties of a photonic crystal so that its' performance can be tuned in degree or kind or even switched on and off reversibly as needed. In a wavelength selective filter, for example, a particular photonic crystal has the ability to select a particular frequency of light from within a broadband input and can preferentially transmit or reflect it. In such an application, it would be desirable to change the performance of the photonic crystal so that it has the ability to select different wavelengths according to the needs of a particular situation. In one situation, for example, it may be desirable to select green wavelengths and in another situation it may desirable to select red wavelengths. When an electromagnetic radiation resonator, as a second example, is coupled to two waveguides, it may enable a frequency selective transfer or routing of electromagnetic radiation from one waveguide to the other. In such an application, it may be desirable to invoke the routing on command to achieve greater control over the propagation of light. It is desirable therefore to have materials such as photonic crystals and devices for controlling the propagation of electromagnetic radiation whose performance is readily tunable or whose effect is reversibly switched on or off.